Voice is no different from piano is no different from any other instrument to the amount of dedicated commitment required to learn it well.
For piano the biggest challenge is to memorise notes. We find it very easy to memorise notes if our hand DOES NOT have to move, but as our hand moves the memory starts to get more difficult. I call it MOVEMENT GROUPS, that is when the hand actually has to move. We can see it often by looking at the notes on the score but often it is very decieving and should be highlighted with color (and beginners/intermediate players have a very tough time observing these). It is these movement groups that we have to practice WITHOUT PAUSING, we can practice 50 hours pausing all the time and we will never be able to play the passage clearly. But if we play slowly and manage to move from one movement group to the next without pause then we are making progress. This is how we hit out at efficiency, perfect practice makes less time for practice and more time for playing. Ineffective practice sticks us in practice mode and depresses us, makes the time required to learn a mystery and keeps us away from playing.
A good way to practice a new piece is to sight read through the entire score, with mistakes and all. This is a good way to measure if the piece will be too hard or not. If we do this and also mark any observations of pattern and general form, we are getting a head start. I would think if we can hear the entire peice we are learning in our heads this also is very important, it makes us rely less on the note values we read and simply we require seeing the dots no so much the tails, again reducing the amount of information needed to read and learn.
I tell students of mine, it is an immediate realisation that you can play a passage right not something that you sleep on and wake up the next day and can do it. We should immediately realise that we can play a passage sucessfully because we find ourselves NOT searching for notes, we have that understanding of where to move before we have to actually move, and the sound produced is clean to our ears. Perhaps things slowly get better and better, our pauses between Movement Groups lesses a little very day, but we can really target this pausing/hesitations specifically and if in our practice session we do this, we will get somewhere immediately! People get too concerned that it is the notes they have to memorise where in fact it is the group of notes requires to be controlled with one hand position and the movement of these groups that they neglect.
So we can put things into a time frame. We cannot say, 20 mins on this will produce the desired result, but we can say, at the end of the week we will complete our note PRACTICE (memorisation and initial control of physical movement[the physical will develop forever into the most effortless touch to produce your desired sound control]) in this part of the music and move on. This is because even when we get our hands around something new, we need to experiment with it, play it for a while until it is completely appreciated (this might take years). Sometimes we have to have some courage to stop being in PRACTICING mode as soon as we get our hands around something. Move on to something else to PRACTICE on but play what you have just learnt over and over again. Keeping things seperate, know when you are PRACTICING and when you are PLAYING. Notice these two differeces, people also often do not do to their disadvantage. Try to PLAY what you PRACTICE as soon as possible and try to PRACTICE new passages as soon as possible. When we Practice we are 100% focusing on the physical movement (although in the back of our heads we are concerned with the ideal sound), when we are playing we are 100% listening to what we are producing (although we still feel the physical excecution but try to keep it all relaxed/effortless to enhance our sound control anywhere we wish).
It is difficult to judge when you are going to stop practicing to PLAY. Practice I guess has 1 true level and 2 Musical levels of practice. The first level is learning the notes, appreciating how the hands move etc, this is not playing, it is purely practice. This is a very unmuiscal level, we try to make our hands understand how to play groups of notes without thought. The two musical levels of practice; the first is forgetting about the physical and focusing compeltely on the sound (which I called PLAYING), and the second level is constant refinements to your physical touch and produced sound (which is also PLAYING). Real practice is really unmusical in my mind, but people can argue that we must practice to play correctly. As we mature musically once the physical is memorised and easy, the musical pours in automatically but constantly pours in and recreates itself, an unending process. It becomes important to understand which level of pratice you are at, and which one you are trying to judge the amount of time with. I feel that the last level of musical practice is never ending but the 1st level of physical practice has a definite time line which can be measured and judged. The 1 musical level of practice, when you finally can listen to yourself play and forget about the physical I think about 1 month of being in this realm is sufficient to get your playing to a good standard (given that the physical is all organised and not a problem)